Hitherto, in electronic parts such as hybrid packages, multimodules, and sealed-type integrated circuits with plastics or metals, with the increase of the integration of IC circuits, the heating value is increased and because there is a possibility of causing the malfunction of electronic parts by the increase of temperature, a countermeasure for preventing the occurrence of the malfunction by attaching a heat-radiating member such as a heat sink, to the electronic part has been taken.
As a method of attaching a heat-radiating member to an electronic part, a method of using an adhesive prepared by adding an aluminum powder to a composition containing a polymerizable acrylic acid ester monomer and a free radical initiator is known as disclosed in U.S. Pat. No. 4,722,960. However, in the method, it is necessary to carry out a heat treatment by using a primer or blocking oxygen after coating the above-described adhesive onto one or both of the electronic part and the heat-radiating member, a long time and much labor are required for the adhering treatment, and it is necessary to temporarily fix the adherend until the adhesive is cured. Thus, the method has a problem in poor production efficiency of electronic devices.
To avoid such a problem, a method of using a so-called heat-conductive pressure-sensitive adhesive having a heat conductivity and a pressure-sensitive adhesive property is known. In the method, the above-described heat-conductive pressure-sensitive adhesive is placed between an electronic part and a heat-radiating member and by utilizing the pressure-sensitive adhesive property, both the members are fixed by adhesion. Thus, the method does not essentially cause the problem of requiring a long time and much labor for the adhering treatment. However, it is considered that, for example, in a certain kind of commercially available heat-conductive pressure-sensitive adhesive tape, an acrylic polymer is used as the adhesive polymer thereof, but in such a heat-conductive pressure-sensitive adhesive tape, there is a difficulty that the pressure-sensitive adhesive tape does not show a good adhesive force to electronic parts and heat-radiating members which are made up of metals having a high polarity, which appears to be attributable to the monomer composition of the above-described acrylic polymer, and is inferior in the adhesive-fixing force between the electronic part and the heat-radiating member.
Also, as other heat-conductive pressure-sensitive adhesive tapes, a pressure-sensitive adhesive tape containing in the pressure-sensitive adhesive layer thereof silver particles having the particle sizes larger than the thickness of the layer (U.S. Pat. No. 4,606,962), a pressure-sensitive adhesive tape wherein heat-conductive electrically insulating particles are randomly dispersed in an acrylic polymer [JP-A-6-88061 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), and EP-566093 A1], etc., are known. These pressure-sensitive adhesive tapes show a relatively good adhesive force to electronic parts having a high polarity but in these tapes, there are problems that the heat-conductive pressure-sensitive adhesive itself is inferior in the stability, that is, during or after the preparation of the pressure-sensitive adhesive, the viscosity thereof is extremely increase to loose the fluidity thereof, whereby the handling property, and, particularly, the coating workability becomes poor, and in the case of forming a pressure-sensitive adhesive tape, the accuracy in the thickness is not obtained as well as, in the worse case, the tape cannot be formed.
As described above, the above-described heat-conductive pressure-sensitive adhesives and the pressure-sensitive tapes using them have the merits that a long time and much labor are not required for the adhering treatment and an electronic part and a heat-radiating member can be easily fixed by adhering, but have the problems that they are inferior in the adhesive-fixing force and are inferior in the stability of the pressure-sensitive adhesives themselves. Accordingly, in the case of applying them to fix electronic parts and heat-radiating members, it has been strongly desired to solve these problems.
In view of these circumstances, an object of the present invention is to provide a heat-conductive pressure-sensitive adhesive being excellent in the heat-conductivity and the adhesive property, particularly, showing a large adhesive force under a high temperature (heat-resisting shear retention force), showing no rapid increase of the viscosity of the pressure-sensitive adhesive during or after the preparation of the adhesive, and also being excellent in the stability and pressure-sensitive adhesive sheets prepared by forming the pressure-sensitive adhesive into sheet forms and tape forms; which are used for fixing electronic parts and heat-radiating members and also for fixing members in other various fields such as building materials, vehicles, aircrafts, and ships.